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transformer. If the AC ripple component of the output is important to the process, then an oscilloscope with a built-in true rms feature can be used to view the ripple waveform, as well as determine the AC to DC ratio of the ripple using the AC and DC coupling of the scope. The electronics are the most complex part of the power supply. Electronic cir- cuits are usually indicated on schematics by boxes with terminal numbers and functions labeled along the edges. The DVM is commonly used in the testing of these electronic circuits to measure signal and control voltages. Although there are many different types of electronic circuits, two are found in every power supply and must function correctly for proper power supply operation. These are the drive circuit and the firing circuit. In some cases, these will be on one circuit board, whereas at other times, they will be on separate boards. The drive circuit is an analog amplifier circuit. It receives current and voltage reference signals from the operators ACC and AVC potentiometers. These con- trol signals will typically range between 0 and 2.5 V DC, depending on the posi- tion of the operator controls. To check a typical drive circuit initially, verify that there is 120 V AC on the power terminals and that there are reference volt- ages on the ACC and AVC input terminals. You should then have a voltage at the output terminals. If no signal is available at these output terminals, the drive cir- cuit may be defective or seriously out of adjustment. Remove and further test the drive circuit using the test procedures found in your operators manual. The firing circuit accepts the output signals of the drive circuit and produces synchronized gate pulses that fire the thyristors in the AC power circuit, which in turn regulates the voltage to the primary of the main power transformer. To test this circuit, ensure there is a signal of more than 2V DC at the input from the fir- ing circuit. Then measure the signals at the gate outputs to the thyristors with the DVM. They should typically be about 1 V DC. Perform these measurements with great care against shorting any of the leads to ground or to another pair of ter- minals, as there may be line voltages of up to 600 V AC between these terminals and ground. As with the drive circuit, if any signals are missing or incorrect, remove the board and bench repair using the procedures outlined in the operators handbook. BASIC REPAIRS Once a defective component has been located, it should be replaced with a part of comparable quality and ratings. It is especially important when replacing tem- perature sensors that the replacement have the same temperature rating as the original. Caution: Before attempting replacement of any component, ensure that the power is removed from the rectifier and that the capacitors are discharged. Electrical and Electronics Components Replacement of electrical components, such as push buttons, thermal switches, relays, and switches, as well as electronic PC boards, is relatively straightfor- ward. Carefully mark all connections to the defective device before removal, replace with the correct item, and reattach the wires. It is also advisable to check the rest of the rectifier for clean and correct connections at this time. Thyristors and Diodes Thyristors are typically found in modular, stud-mount, and flat-pack configu- rations, whereas diodes are usually the stud-mount or flat-pack style. The 646